1. Field of the Invention
This invention relates to a heat receiver tube and a method for manufacturing the heat receiver tube. More over a parabolic trough collector with the heat receiver tube and a use of the parabolic trough collector are provided.
2. Description of the Related Art
A sun energy collecting unit of a sun field power plant based on the concentrated solar power technique comprises for instance a parabolic trough collector with at least one parabolic mirror and at least one heat receiver tube. The heat receiver tube is arranged in a focal line of the mirror. By sunlight reflecting surfaces of the mirror the sunlight is focused to the heat receiver tube, which is filled with a heat transfer fluid, e.g. a thermo-oil or molten salt. Via the heat receiver tube the energy of the sunlight is coupled into the heat transfer fluid. Solar energy is converted to thermal energy.
In order to maximize an efficiency, with which the energy of the sunlight is coupled into the heat transfer fluid, a solar energy absorptive coating is attached on a core tube surface of the heat receiver tube. Such an absorptive coating commonly comprises a multilayer stack with sequentially deposited thin film layers having different optical characteristics. For instance, the depositing the thin film layers are carried out by a sputtering method.
An essential overall optical characteristic of the absorptive coating is a high solar absorbance (low solar reflectivity) for wavelengths of solar spectrum (absorption radiation). Additionally a low emissivity (high reflectivity) for infrared radiation is essential, too. A coating with these optical characteristics is called selective solar coating.